Method for drying or other treatment of a web-like material



F. SVAVAR ET AL METHOD FOR DRYING OR OTHER TREATMENT OF A WEB-LIKE MATERIAL Filed June 23, 1954 June 7, 1960 DISTANCE OF WEB FROM STARTING POINT F1 2 {WW Rune Gusgsow 5y We United States Patent METHOD FOR DRYING OR OTHER TREATMENT OF A WEB-LEE MATERIAL Filip Svavar and Rune Gustafsson, Stockholm, and Torsten Waern, Stockholm-Bandhagen, Sweden, assignors tso igkhebolaget Svenska Flaktfabriken, Stockholm,

we on Filed June 23, 1954, Ser. No. 438,728

Claims priority, application Sweden June 30, 1953 3 Claims. (Cl. 34-18) The present invention relates to a method for drying or similar treatment of a web-like material, which during the treatment, is carried on one or more heated, preferably cylindrical, surfaces, and at the same time is exposed to a treating medium.

In order to increase the capacity when treating materials such as pulp or the like on heated surfaces, it is known that the treatment is improved by directing a gaseous treating medium against the material while it is in contact with the heated surface. The present invention is based on the fact that a substantially increased treating effect may be achieved by supplying the treating medium under controlled conditions so that the different stages of the treatment take place according to the different physical conditions of the material. It has been proved that by treating pulp or similar hygroscopic materials on heated surfaces, the intensity of evaporation varies in a predetermined manner at different points in the path of travel of the material on the heated surfaces.

Referring to the accompanying evaporation curve in Fig. l, the distance AB represents the part of the path where the fibres and liquids are heated to the wet bulb temperature of the treating medium. The distance B--C in the curve represents the part of the path where the evaporation is substantially constant, as from a wet surface, whereas the distance CD represents part of the path where the evaporation takes place with decreasing intensity. The change in the evaporation curve at the point C is due to the fact that during the period B--C, the heat conduction through the material and the heat transmission to the same, are dominating. After the material reaches its fibre saturation state at point C, some fibres of the material have reached their final condition and there is no longer unbroken flow of liquid through the material such as occurs during the period B-C. After the fibre saturation state is reached, the diffusion of the liquid within the body of the web, is of decisive importance.

The present invention relates to an improved method for drying or similar treatment of a hygroscopic material of the type mentioned above wherein the method is accomplished by controlling the condition of the treating medium supplied to the material in the stages before and after the material reaches its fibre saturation state differently so as to correspond to the above-mentioned variations in the evaporation intensity during such stages.

The invention is characterized in that the treatment is controlled to provide at least one stage before the fibre saturation point is passed and one stage after said point, and that the treatment medium is caused to circulate in the first stage with a certain amount being discharged to open air, while a corresponding quantity of the treatment medium is conducted from the second stage and mixed with the recirculated portion of the medium from the first stage prior to introducing the mixture into the first stage. An equivalent quantity of medium 1s supplied from the outside to the second stage in order to make up the quantity transferred to the first stage.

Further characteristics of the invention will be evident 2,939,222 Patented June 7, 1960 ice from the claims and the following description of an apparatus for performing this method.

The invention is hereinafter more fully set forth in detail with reference to the accompanying drawing wherein:

F g. 1 illustrates an evaporation curve in which the vertical axis represents the intensity of evaporation and the horizontal axis represents the distance of the material from the starting point; and

Fig. 2 illustrates a suitable embodiment of an apparatus for performing the method of the present invention.

In the drawing, a web 1 which is wetter than its fibre saturation state is advanced onto a heated surface 3 and after treatment, is withdrawn in a condition dryer than its fibre saturation state, as indicated at 33. In the illustrated embodiment, only one heated surface is shown, but there is no obstacle for treating the material progressively on -a plurality of heated surfaces. Distributing boxes 6 and 29 extend over the entire width of the heated surface 3. The boxes 6 adjacent the inlet end of the apparatus are provided with slots 4 or perforations for supplying the treating medium to the web while it is supported on the heated surface 3. The intermediate spaces 5 between the boxes are utilized for removing and exhausting the treating medium after being used. The used treating medium is collected in the space 8 which is formed by the intermediate space between the outer casing 9a and the distributing boxes 6. The treating medium is discharged through a duct 34 and is partially exhausted by means of a fan 10 and partially recirculated by a fan 12. The treating medium exhausted by the fan 10 is either conducted away directly to open air or is passed through one or more heat exchangers, the different parts of which are designated 13, 14, 15, 16, and 17. Between the duct 34 and the inlet of the fan 12, a damper 11 is provided for adjusting the proportion between the recirculated and exhausted portions of the treating medium. In Fig. 2, the points A, B, C, and D correspond to the points shown in the evaporation curve of Fig. 1. By means of the fan 12, the treating medium is supplied through the duct 20 and through a heat exchanger 18 and damper 19, and then introduced by means of the distributing boxes 6 to the part of the path of travel corresponding to the distance A-C in the curve of Fig. 1, where C indicates the point where the material reaches the fibre saturation state. The medium is controlled to insure that the material reaches the fibre saturation state at point C.

In the other part of the path of travel of the web, i.e. the distance CD in the diagram of Fig. l, the treating medium is supplied in the same way by means of the distributing boxes 29 provided with openings 30 and arranged so that the treating medium used in this part of the path of travel is removed and exhausted through intermediate spaces 31 between the boxes. The used treating medium is collected in a space 28 formed by the intermediate space between the casing 9b and the boxes 29. From this space, the used treating medium, to a substantial extent, is conducted through a duct 35 to a fan 12. Some of the treating medium introduced to and removed from the web in the path CD, may be recirculated via a damper 21 and a duct 23 to a fan 24 for recirculation. This part of the treating medium following the fan 24 is caused to pass through a heat exchanger 25 and a damper 27 and is then introduced through a duct 26 to the distributing boxes 29 arranged over the path C-D of the web, i.e. the part of the path of the web situated between the fibre saturation state C and the end D of the treatment. The medium is controlled to cause the material to reach the desired final condition at the point D. In the duct 23, the treating medium recirculated in this part of the process, may be mixed with the necessary amount of fresh treating medium supplied from the outside, either as an 3 a ungoiligitioned medium or from the heat exchangers 13 an While a particular embodiment of the present invention has been herein illustrated and described, it is not intended to limit the invention to such disclosure, but changes or modifications may be made therein and thereto within the scope ofthe following claims.

What we claim is: v

1. The method of drying a hygroscopic web from a condition wetter than its fibre saturation state to a desired condition drier than its fibre saturation state comprising the steps of introducing the wet web onto a rotating heated cylindrical surface through a treatment zone to heat the web by conduction through one surface, drying the heated the other surface, controlling the medium in the first stage to dry the heated web by evaporation from the surface to cause the web to reach fibre saturation state at the end of the first stagelof drying, subjecting the Web in the second stage of. the zone to a second gaseous drying medium through said other surface, controlling the second medium to dry the heated web by difiusion in accordance with the hydroscopic properties of the web to cause the web to reach said desired condition at the end of the second stage of drying, exhausting the first and second gaseous mediums from the respective stages of the treatment Zone after subjecting the web material thereto, conducting the said first and second gaseous mediums in a generally counter-current direction to the travel of the web material by discharging a given quantity comprising at: least a. portion of said first gaseous medium to atmosphere, introducing a like quantity comprising at least a portion of said second gaseous medium into said first gaseous medium prior to entering the first stage of the treatment zone, and supplying a corresponding quantity of outside air to said second gaseous medium prior to entering the second stage of the treatment zone.

2. The method according to claim 1 wherein the first and second gaseous drying mediums are introduced into the treatment zone at a plurality of areas located at spaced intervals longitudinally of the web and extending transversely of the width thereof and are exhausted from the treatment zone outwardly through the spaces between the longitudinally spaced areaswhereby the web throughout each stage is treated by drying medium at a condition controlled in accordance with the condition of the web relative to its fibre saturation state.

3. The method according to claim 1 wherein the control of the second gaseous medium provides'a drying power in said medium greater than the drying power in said first gaseous medium.

References Cited in the file of this patent UNITED STATES PATENTS 1,757,948 Hockley May 6, 1930 2,107,275 Anderson et a1. Feb. 8, 1938 2,268,988 Hess et a1 Jan. 6, 1942 FOREIGN PATENTS 354,099 Great Britain Aug. 6, 1931 79,632 Sweden Feb. 20, 1934 652,038 Germany Oct. 23, 1937 674,969 Germany Apr. 26, 1939 261,636 Switzerland Sept. 1, 1949 

